Path Traversal in Microsoft Windows

CVE-2026-39844

NiceGUI is a Python-based UI framework. Prior to 3.10.0, Since PurePosixPath only recognizes forward slashes (/) as path separators, an attacker can bypass this sanitization on Windows by using backslashes (\) in the upload filename. Applications that construct file paths using file.name (a pattern demonstrated in NiceGUI's bundled examples) are vulnerable to arbitrary file write on Windows. This vulnerability is fixed in 3.10.0.

Vulnerability class: Path Traversal (Directory Traversal)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.005 (39.5th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 40th percentile — 39.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-39844: held from 0.004 to 0.005.

EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-39844?
CVE-2026-39844 is a medium-severity vulnerability in Microsoft Windows, classified under Path Traversal. CVSS score: 5.9/10. Published 2026-04-08.
How severe is CVE-2026-39844?
Medium severity. CVSS v3 base score is 5.9 out of 10.